A power converter including a PWM rectifying circuit, a PWM inverter circuit, and a common mode filter including a common mode choke coil and a capacitor has conventionally been presented (cf. Patent document 1).
FIG. 1 is an electric circuit diagram showing a motor driving system for driving a motor via a power converter formed by a diode rectifying circuit and a PWM inverter circuit.
This motor driving system rectifies a three-phase ac voltage to a dc voltage in the diode rectifying circuit, and carries out pulse width modulation by switching devices of the PWM inverter circuit by comparing a triangular wave signal which is a carrier wave and a modulation wave, to thereby output an ac voltage of a desired voltage and frequency and supply it to the motor. Then, when the respective switching devices of the PWM inverter circuit operate, a common mode voltage Vinv {cf. (E) in FIG. 2} is generated as shown in FIG. 2. Defining potentials between output terminals U, V, W of the PWM inverter circuit and a middle point N of a dc part as Vun, Vvn and Vwn, the common mode voltage Vinv is expressed as:Vinv=(Vun+Vvn+Vwn)/3which takes four values of Ed/2, Ed/6, −Ed/6 and −Ed/2, and presents a step-like waveform having a carrier frequency of the PWM inverter as the fundamental component.
This common mode voltage Vinv results in harmonic leakage current and a shaft voltage when driving a motor by an inverter. The harmonic leakage current results in a conduction noise, which mainly causes a noise terminal voltage. Therefore, various countermeasures are studied to solve problems resulting from this common mode voltage Vinv.
Of these countermeasures, a method of reducing harmonic leakage current using a common mode filter formed by combination of a common mode choke coil and a capacitor is generally used. An equivalent circuit to the common mode of a motor driving system using a common mode filter is as shown in FIG. 3. A common mode choke coil for use in the common mode filter has a magnetic material core with three-phase windings wound therearound such that they are equal in polarity and number of turns, and has an inductance of zero for a normal mode since magnetomotive forces caused by three-phase currents are canceled out, while operating as a great reactor against the common mode. However, when the magnetic flux density of the common mode choke coil exceeds a saturation magnetic flux density Bmax, the inductance is reduced drastically, so that it no longer serves as a common mode filter.
Herein, defining a voltage applied to the common mode choke coil as VLC1 and the number of turns of the common mode choke coil is N, a magnetic flux φLC1 of the core is expressed by Equation 1, and defining the effective cross section of the magnetic material core as S, a magnetic flux density BLC1 is expressed by Equation 2.
                              ϕ          ⁢                                          ⁢          LC          ⁢                                          ⁢          1                =                              1            N                    ·                      ∫                          VLC              ⁢                                                          ⁢              1              ⁢                              ⅆ                t                                                                        [                  Equation          ⁢                                          ⁢          1                ]                                          BLC          ⁢                                          ⁢          1                =                                            Φ              ⁢                                                          ⁢              LC              ⁢                                                          ⁢              1                        S                    =                                    1              SN                        ·                          ∫                              VLC                ⁢                                                                  ⁢                1                ⁢                                  ⅆ                  t                                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
And, the absolute value of the common mode voltage rises to maximum in the case where all of three-phase arms of the inverter are turned on at the positive side or negative side (where the inverter has a modulation factor of zero). Herein, a flux linkage φinv is expressed by Equation 3 defining a carrier period of the inverter as Ti and in the case where the common mode voltage Vin is applied as a whole to the common mode choke coil, which increases when a dc voltage Ed is high or the switching period is long, so that the common mode choke coil is more likely to bring about magnetic saturation.
                              ϕ          ⁢                                          ⁢          inv                =                                            1              N                        ·                          ∫                              Vinv                ⁢                                  ⅆ                  t                                                              =                                    Ed              ·              Ti                                      8              ·              N                                                          [                  Equation          ⁢                                          ⁢          3                ]            
And, in order to reduce the magnetic flux density such that the common mode choke coil does not saturate, the cross section S of the coil needs to be increased or the number of turns N needs to be increased. That is, in either case, the core is increased in size. FIG. 4 shows a carrier wave, each phase voltage, a common mode voltage Vinv and a flux linkage φinv {cf. (A) in FIG. 4} of the common mode choke coil when a modulation factor Ki of the inverter is 1, and a carrier wave, each phase voltage, a common mode voltage Vinv and a flux linkage φinv {cf. (B) in FIG. 4} of the common mode choke coil when the modulation factor is 0.
Further, to comply with harmonic current regulations, a PWM rectifying circuit (cf. FIG. 5) having switching devices provided in parallel with a diode is used in some cases. In this case, the switching devices of the PWM rectifying circuit also carry out the operation similar to the PWM inverter circuit, which causes a common mode voltage Vrec to be generated from the PWM rectifying circuit.
An equivalent circuit to the common mode of the PWM rectifying circuit and PWM inverter circuit is as shown in FIG. 6, where the common mode voltages Vrec and Vinv generated by the PWM rectifying circuit and PWM inverter circuit for the dc link are in reverse series.
And, in the case where the carrier wave of the PWM rectifying circuit and PWM inverter circuit is made common (in the case where the switching frequency is made equal to each other), the common mode voltages of the PWM rectifying circuit and PWM inverter circuit cancel each other as shown in FIG. 7, causing a common mode voltage Vcc as a whole to be smaller than the common mode voltages generated by the respective PWM rectifying circuit and PWM inverter circuit.    Patent document 1: Japanese Patent Application Laid-Open No. 2003-18853